Aircraft generators are typically driven by variable-speed aircraft engines, such as for example by turbo-jet, turbo-fan, and prop-fan engines. In such systems the engine speed varies over the ground/flight envelope, which involves ground-taxiing, takeoff, cruise and landing phases of the aircraft. When the engine drives an electric-generator, the generator, turning at a variable speed, then puts out variable frequency power. Previous aircraft design, desiring a 100% conditioned constant frequency power output, typically used a 3-phase 400 Hz 200 VAC generator, driven by a constant speed drive (CSD) unit (interposed between the engine and the generator), or from an electronic converter system powered from a direct driven variable frequency generator.
One of the most widely used devices for providing 100% conditioned electric power has been the hydraulic CSD unit. This unit typically comprises a variable displacement hydraulic pump or a fixed displacement hydraulic motor that operates with a differential planetary gear system to maintain a constant rpm to the generator, over a requisite input speed range. The CSD unit which is placed between the engine variable speed drive and the generator, are mechanisms whose internal working parts are subjected to high hertz stresses, with consequent wear and heat dissipation problems. The service life and overhaul cycle of such systems are relatively short and considerable maintenance thereof is required. Finally, in addition to high production costs, the prospective higher-power limitations of these systems tend to inhibit the development of acceptable light weight systems. An example of a CSD type device mounted between an aircraft engine drive and a generator to provide ac current at a regulated frequency can be found in U.S. Pat. No. 2,500,655 to Bevins et al. A variation of this approach to constant power (100%) production is the "integrated drive generator" (IDG), which integrates the CSD and generator in one unit, thus requiring only a single mount and cooling interface with the dedicated cooling system, that must be provided for the CSD systems.
Another approach to providing constant-frequency power has been the use of advanced electric power system technology in systems known as VSCF (variable speed constant frequency) systems. Under such systems the generators are driven directly by the engines and are, therefore, subject to the engine speed variations. This results in generation of variable-frequency electric power which is then converted to constant-frequency electric power by static (electronic) power converters/inverters. These VSCF systems have typically used "cycloconverter", "dc link" or "270 VDC" technology.
The cycloconverter approach comprises a system for demodulating a lower frequency wave from a higher frequency wave and typically utilizes a static power converter, including a plurality of SCR devices. The lower synthesized ac wave is then passed through a filter to yield the constant frequency power desired, typically 400 Hz power. Such cycloconversion VSCF systems are depicted in U.S. Pat. No. 3,585,485 to Gywgyiand U.S. Pat. No. 3,141,418 to Plette. The power conditioning system in the '485 patent includes an improved firing angle control over the thyristor type converters and cycloconverters in the system. Yet another type of cycloconversion VSCF system is depicted in U.S. Pat. No. 4,038,592 to Stummer.
One problem with the cycloconversion VSCF system is that to generate the high quality 400 Hz power desired, the generator must turn at a high speed to produce a high enough frequency to be demodulated: this typically is three to four times higher than the 400 Hz power desired. This dictates the need for the frequency in the order of 1600 Hz at base speed. Since the range of an engine is usually 2:1 then if the generator develops 1600 Hz power at 50% speed, it will generate 3200 Hz at full speed. These high frequencies require high speeds of (typically) 10,000 to 20,000 rpm and a large number of (electrical) poles.
Another type of VSCF system is the "dc-link" power conditioning system. In this type of system variable frequency power supplied by a variable speed generator is rectified and inverted into constant frequency power for supply to aircraft electrically powered sub-systems. This type of approach is depicted in U.S. Pat. No. 2,892,098 to Bergvall, wherein power is conditioned into constant frequency ac and dc power. Under the system in the '098 patent, generators driven directly by the aircraft engines have a variable frequency alternating current output which will simultaneously supply direct current and constant frequency alternating current by utilizing a system of semiconductor rectifiers and static inverters.
Yet another type of VSCF power supply is a system which may be characterized as a 270 VDC system. In such a system a bridge rectifier is placed within the generator housing to rectify the 3-phase variable-frequency ac power to 270 VDC. Under this type of system it is necessary to invert a large percentage of the power for motor loads and for the large amount of 400 Hz equipment. This incurs significant weight disadvantage in some aircraft.
Other prior art approaches to VSCF systems for converting indeterminate variable engine shaft speed to a conditioned electrical output having a chosen value or range of determinate frequencies are exemplified in U.S. Pat. Nos. Re. 26,630 and Re. 26,327 to Peaslee, 2,899,566 to Ware et al, 2,967,252 to Blake, 3,274,482 to Depenbrook, and 3,641,418 to Plette. The problem common to all of the above VSCF electric power conversion systems is that 100% of the generator output is converted to constant frequency power, and as a result, these systems, like the CSD systems, are heavy and not weight-competitive with the system described herein. The conventional systems are also complex, costly and require a high level of maintenance activity. Additionally, the lower transmission efficiency and the thermal management problems, make the foregoing systems less acceptable.
A system utilizing multiple generators on a single shaft or on shafts driven by a single power source is shown in U.S. Pat. No. 2,026,474 to Kranz. The '474 patent describes a turbine-generator system adapted to meet fluctuating electric arc furnace loads. The system utilizes two or more generators having the rotors thereof driven by counterflow turbine shafts and includes means, including a dc exciter connected in series with the field windings of the generators, to maintain the ratio of the power outputs of the separate generators substantially constant.
All of the prior art systems discussed hereinabove, incorporated by reference herein, involve power systems which are of less than optimum design when used in aircraft power supply systems: particularly those of large capacity. It is a primary object of the present invention to provide an aircraft electrical power system which is optimized in terms of minimizing weight, complexity and cost, which provides only the amount of conditioned secondary power, required by certain aircraft electrical sub-systems, while simultaneously providing primary unconditioned electric power to the remaining electrically powered sub-systems. This described system is also particularly appropriate to future aircraft, where the electric power demands of loads, such as the ECS (environmental control systems), may be very large.